MECHANICAL AND MICROSTRUCTURAL CHARACTERISTICS OF MODIFIED SULFUR POLYMER COMPOSITES

Department of Urban and Environmental Engineering (Urban Infrastructure Engineering)Modified sulfur polymer composite has been developed to replace hydraulic cement concrete in specific applications. Because it has superior properties including the rapid development of high compressive strength, resistance to water environments, and resistance to strong acid and saline attack compared with the hydraulic cement concrete. Most of all, the fabrication of modified sulfur composites excludes water due to the thermoplastic properties of modified sulfur. Thus, all the raw materials such as aggregate and micro-fillers can be easily mixed with plastic modified sulfur to produce the modified sulfur composite. Early in the development of sulfur composites, however, elemental sulfur mostly from the distillation of crude oil in petroleum refineries was used as a binder. The composites made of elemental sulfur presented not only severe curing contraction but also inferior durability in weathering and chemical environments. Considering the aforesaid problems, many researchers developed modified sulfur polymers by reacting the elemental sulfur with a variety of chemical additives at a certain reaction temperature and time. The modified sulfur polymers had alternating compositions according to the reaction conditions such as the types of chemical additive, reaction temperature, and reaction time. Through a modification process, the composition of modified sulfur mixture was composed of abundant polysulfide products along with a reduced free elemental sulfur such as orthorhombic, monoclinic, and amorphous sulfur chains. Such a converted composition considerably enhanced the mechanical and durability properties of sulfur composites. The term, modified sulfur polymer, indicates the final reaction product from a mixture of elemental sulfur and chemical additive. If a certain micro-filler accounts for a part of the modified sulfur polymer, the mixture is usually called as sulfur polymer cement. Among various modified sulfur polymers, this study employed a dicyclopentadiene (DCPD)-modified sulfur polymer as a binder, which is one of the commercially available products. Based on this modified sulfur polymer, total five research topics were carried out step by step in this study. First, different proportions of modified sulfur composites were developed by replacing a portion of the modified sulfur polymer by fly ash and rubber powder. Both the fly ash and the rubber powder were employed to be a substitute for fine aggregate and to enhance the mechanical properties of modified sulfur composites by reducing thermal curing shrinkage. An increasing portion of the fly ash of up to 50 vol.% resulted in a continuous rise of compressive strengths with a given portion of rubber powder. Moreover, the rubber powder also significantly reduced the unit weight of modified sulfur composites without sacrificing the compressive strength. Finally, a series of microstructural analysis suggested the rationales for the enhanced mechanical properties in terms of crystalline phase transition, morphological transition, and porosity. Second, in a similar way as the first research topic, modified sulfur composites included rubber powder, and a blend of Portland cement and fly ash as the binary cement that rendered the different particle characteristics (i.e., particle shape, particle size distribution) of micro-fillers as compared with sole fly ash or Portland cement. An increasing portion of the binary cement generally induced the higher compressive strengths of the modified sulfur composites than those with only fly ash. This was likely due to the larger indentation modulus of Portland cement than fly ash. In addition, the use of rubber powder contributed to a reduced unit weight of sulfur composites without a severe strength reduction. Most of all, because all the crystalline phases from the binary cement remained intact in hardened sulfur composites, the feasibility of using the binary cement as a self-healing material for cracked sulfur composites was confirmed empirically. Third, considering the brittleness of modified sulfur composites except those containing rubber powder identified in the first and second research topics, a series of fiber-reinforced sulfur composites were developed and examined to convert the brittleness of modified sulfur composites into a more ductile manner, and to acquire multiple micro-cracks especially under flexures. Two micro fibers including steel and electrical chemical resistant (ECR) glass fiber were incorporated in the mixtures. By varying the total fraction of micro fibers and adjusting the relative volumetric ratios between steel and glass fiber, the flexural stress-strain responses of modified sulfur composites were greatly modified, which was supported by the change of porosity and the uniform strain distribution revealed by a digital image correlation (DIC). Fourth, the combined effect of particle characteristics of micro-fillers and temperature on the rheological properties of fresh sulfur composites was examined through a rheometer test. Because both the portion of micro-filler and mixing temperature have been critical in determining the workability of modified sulfur composites, the author adjusted the surface areas of a given portion of binary cement and set the temperature at 120??? or 140???. Overall, both the yield stress and plastic viscosity of the modified sulfur composites were higher in 140??? than 120???. Especially, the result of mini slump flow of fresh modified sulfur composites was compared with the rheology test results at 140???. Thus, the results from the rheology and the mini slump test were deemed to suggest an optimal temperature range favorable for placement. Finally, considering the feasibility of using cementitious materials as self-healing materials in the sulfur composites, total eight mix proportions of self-healing modified sulfur composites were developed and examined. The modified sulfur composites were comprised of a binary cement of calcium sulfoaluminate (CSA) expansive agent and Portland cement, superabsorbent polymer (SAP) powder, and fine aggregate. After through crack was made on each sample, all the samples were exposed to two wet environments: one was water curing, and the other was water permeability test. Each of them was dedicated to building different self-healing conditions. While the water curing gave a stable self-healing condition to the cracked samples, the water permeability test without any water curing was analogous to a real water intrusion through the cracks of a certain structure. For each sample, the extent of self-healing was monitored and evaluated by the optical microscope images of surface crack and elastic wave tests, respectively at specified ages. Moreover, computed tomography was used to confirm the recovery of inner crack width after 7 days of water curing. Through a series of tests, it was revealed that a higher ratio of CSA expansive agent than Portland cement in a binary cement promoted the self-healing of through cracks further with the swelling of SAP particles on crack surfaces.clos

Combined Effects of Set Retarders and Polymer Powder on the Properties of Calcium Sulfoaluminate Blended Cement Systems

This study investigates the effects of set retarders on the properties of polymer-modified calcium sulfoaluminate (CSA) and Portland cement blend systems at early and long-term ages. The fast setting of the cement blend systems is typically adjusted by using retarders to ensure an adequate workability. However, how the addition of retarders influences the age-dependent characteristics of the cement blend systems was rarely investigated. This study particularly examines the effects of retarders on the microstructure and strength development of polymer-modified CSA and Portland cement blend pastes and mortars from 2 h to 90 days. The macro- and microstructural properties are characterized by compression testing, powder X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy with energy dispersive spectroscopy. The test results reveal that the use of retarders delayed the strength development of the cement blend systems at the very early age by hindering the production of ettringite, which was cumulative to the delaying effect of polymer, but it increased the ultimate strength by creating denser and finer pore structures with the evolution of hydration products

Rheological properties of cement pastes with cellulose microfibers

The rheological properties of cement pastes prepared using kenaf-plant cellulose microfibers (CMFs), which was incorporated for the purpose of internal curing, were investigated. The main test variables are the length and mass fraction of the CMFs. CMFs of lengths of 400 ??m and 5 mm were included in the mixtures at 0.3, 0.6, 1, and 2 wt.% of the cement. Dry CMF particles were wetted with water to the fiber saturation point using vacuum filtration immediately before mixing. An optimum shearing protocol was designed to minimize the shear-induced structural breakdown of cement pastes with the CMFs under hysteresis loops of the shear strain rate. It consisted of an initial pre-shearing step at a high shear strain rate of 80 1/s, three acceleration and deceleration cycles with a maximum shear strain rate of 40 1/s, and a rest step before each acceleration. The mixtures??? flow curves were well fitted to the Herschel???Bulkley fluid model with a minimum coefficient of determination of 0.9993. The yield stress of cement pastes was at least 34.3% higher for longer CMFs at the same dose. However, the mixtures exhibited similar plastic viscosities with a coefficient of variation of only approximately 5.8%

Applicability of Diffuse Ultrasound to Evaluation of the Water Permeability and Chloride Ion Penetrability of Cracked Concrete

This study aims to explore the applicability of diffuse ultrasound to the evaluation of water permeability and chloride ion penetrability of cracked concrete. Lab-scale experiments were conducted on disk-shaped concrete specimens, each having a different width of a penetrating crack that was generated by splitting tension along the centerline. The average crack width of each specimen was determined using an image binarization technique. The diffuse ultrasound test employed signals in the frequency range of 200 to 440 kHz. The water flow rate was measured using a constant water-head permeability method, and the chloride diffusion coefficient was determined using a modified steady-state migration method. Then, the effects of crack width on the diffusion characteristics of ultrasound (i.e., diffusivity, dissipation), water flow rate, and chloride diffusion coefficient are investigated. The correlations between the water flow rate and diffuse ultrasound parameters, and between the chloride diffusion coefficient and diffuse ultrasound parameters, are examined. The results suggest that diffuse ultrasound is a promising method for assessing the water permeability and chloride ion penetrability of cracked concrete

Characteristics of GGBFS-Based Pervious Concrete Considering Rheological Properties of the Binder

To mitigate environmental challenges, such as urban flooding, noise pollution, and the urban heat island effect, pervious concrete has been developed. This research was intended to develop pervious concrete made from ground granulated blast furnace slag (GGBFS) to further decrease the environmental impact of the construction sector by reducing the content of ordinary Portland cement (OPC). The primary objective of the mix proportion was to maximize water permeability while meeting the required compressive strength. Two levels (60 and 100%) of OPC replacement by GGBFS were evaluated and compared to OPC-only concrete, and two target porosities (10 and 15%) were achieved by modifying the binder-to-aggregate ratio. CaO and CaCl2 were utilized as an activator and an accelerator, respectively, for the GGBFS only binder. Characteristics of the pervious concrete were determined with the compressive strength, porosity and water permeability test. Meanwhile, the effects of the rheological properties of binders on the water permeability and compressive strength of pervious concretes was evaluated. According to the results, the permeability of pervious concretes always exhibited a positive correlation with porosity, regardless of binder type. Although, the pervious concrete made with CaO-activated GGBFS has a lower compressive strength than the other two cases (60% GGBFS and only OPC), it still meets the minimum strength requirement. Based on the rheology studies of binder, it was found that, the adhesion force of the binder and the compressive strength of the pervious concrete decreases, as evaluated by rheology studies on binders. The CT scan revealed that when the adhesive force of the binder was weaker, the local porosity was higher (i.e., pore volume was larger) at the bottom of the specimen, which might be due to the limited consolidation and compaction of the binder between aggregate particles at the bottom due to its higher plastic viscosity

Solidification of waste rubber and fly ash using modified sulfur polymer

This study utilized several surplus industrial wastes in making non-cementitious construction materials. The main objective of this study is to develop modified sulfur composites that possess superior sustainability to Portland cement concrete (PCC). Dicyclopentadiene (DCPD) modified sulfur was used as a binder with thermoplastic property. The rubber powder and micro-filler (fly ash and cement) were used as the additives in this study. Fundamental physical properties of modified sulphur composites were investigated. Several microstructural analyses (e.g., scanning electron microscopy, X-ray diffraction) were conducted

Direct-tensile and flexural strength and toughness of high-strength fiber-reinforced cement composites with different steel fibers

The main purpose of this study is to investigate effects of the type and volume fraction of steel fibers on the mechanical behaviors of fiber-reinforced cement composites (FRCCs) with high strengths. Various FRCC mix cases were designed and tested in two steps. At the 1st step, two types of steel fibers (straight and hooked) were examined with two different fiber volumes (1.0 and 1.5%). The test variables of the 1st step included the type and volume fraction of steel fibers, and the inclusion of coarse aggregate. At the 2nd step, ultrahigh strength FRCC mix cases using hooked steel fibers only were tested with two different fiber volumes (1.0 and 1.5%). Various mechanical tests were performed to evaluate the stress-strain behaviors of the FRCCs subjected to uniaxial compression, direct tension, and third-point bending. The test results reveal that the use of hooked steel fibers improved the tensile and flexural capacities of FRCCs more effectively than did the straight steel fibers. Also, the FRCCs generally demonstrated greater direct-tensile and flexural toughness compared with ordinary fiber reinforced concretes (FRCs).clos